In a copending application of Norman L. Stauffer, Ser. No. 912,688, filed June 5, 1978, now U.S. Pat. No. 4,185,191, a range determination system is shown which is useable in a through-the-lens camera for positioning the taking lens at a desired correct focus position with respect to a remote object within the field of view. That system utilizes a plurality of small detectors such as charge coupled devices (CCD) or charge injection devices (CID) mounted in an array to receive radiation from the scene being viewed. The detectors are arranged in pairs with each pair being mounted behind a small lenslet so that each pair receives a view of the exit pupil of the taking lens but one of the detectors in each pair receives radiation primarily from a first portion of the taking lens while the other of the detectors in each pair receives radiation primarily from a different portion of the taking lens. The result is to create two similar curves indicative of the radiation distribution pattern from the scene being viewed. At the proper focus position the two curves coincide, but as the object changes position with respect to the camera, the two curves move with respect to each other to indicate an out-of-focus situation. The two curves move with respect to each other in a first direction when the object is closer to the camera than the desired focus position and move in an opposite direction with respect to each other as the object moves further from the camera than the desired focus position. By determining the direction of movement of the two curves with respect to each other, it is therefore possible to determine the direction that the taking lens has to be moved to achieve the desired focus position.
In a copending application of Norman L. Stauffer and Dennis J. Wilwerding, Ser. No. 16,595, filed Mar. 1, 1979, an improvement on the above-mentioned copending application Ser. No. 912,688 was disclosed wherein a method and apparatus for determining the direction in which the two similar curves need to be moved in order to produce the coincidence indicative of a proper focus position is disclosed. In that application a value indicative of the slope of the curves at predetermined points in obtained and this value is multiplied by the difference in the value of the output of the detectors at such points. The product is summed over a predetermined range. The summation value is substantially zero when the two curves coincide but will have a characteristic such as plus or minus to indicate the direction the taking lens must be moved when the two curves do not coincide.
It has been found that the apparatus for multiplying the signal indicative of slope by the signal indicative of difference is more complicated and costly than desired in a system in which cost, complexity and weight are important factors (such as in hand-held cameras).